Twin-wire presses for dewatering of fiber suspensions and forming a continuous web thereof are previously known. Dewatering of the pulp is usually from an inlet pulp concentration of from 3-8 percent by weight to outlet pulp concentrations of 30-50 percent by weight. According to the state of the art, such twin-wire presses comprises lower rolls, an endless lower wire running in a path around the lower rolls, upper rolls, and an endless upper wire running in a path around the upper rolls. The two wires co-operate with each other along sections of these paths that run substantially in parallel with each other for dewatering of the fiber suspension between the wires during displacement thereof. An inlet box provides for supply of the fiber suspension to a wedge-shaped dewatering space between the wires. The twin-wire press further comprises two dewatering tables supporting the respective wire in these sections of the path and forming the wedge-shaped dewatering space between the wires for initially pressing and dewatering the fiber suspension, whereby a web is formed between the wires, and a roll arrangement situated after the dewatering tables in those sections of the paths, as seen in the direction of movement of the wires, for finally pressing and dewatering the web between the wires, so that the web will obtain a desired dryness. By dewatering space is meant the section between the dewatering tables where dewatering occurs. Alongside the longitudinal direction of the wires, in the wedge-shaped dewatering space, there are perforated dewatering elements that are arranged against the wires outside the dewatering space, through which dewatered filtrate is lead away to upper and lower outlet boxes, respectively, arranged at the dewatering tables for receiving filtrate that flows from the dewatering space through the upper and lower wires, respectively. The dewatering elements which rest against the upper and lower wires, respectively, in the wedge zone constitute together an upper and lower dewatering surface, respectively, where each dewatering surface may be composed of one or more dewatering elements. Upper and lower outlet boxes may be divided into several chambers whereby a filtrate through the upper and lower dewatering surfaces, respectively, may be divided into partial filtrate in two or more chambers in respective outlet boxes.
A traditional dewatering space in a twin-wire press has a wedge-shape with a fixed design that is not changeable when the twin-wire press is in operation. The outlet boxes are not sealed, and thus work against atmospheric pressure. The geometry of the table and the pulp suspension flow creates the operating pressure difference over the wire that controls the dewatering. The wedge shape determines the pressure build-up in the twin-wire press, and the dewatering process is to a large extent dependant on the shape of wedge, which is difficult to change. Changes of the wedge-shape requires new, extensive settings of the dewatering tables, a change of side sealings to the dewatering tables, etc. Owing to the fact that the wedge-shaped dewatering space has its largest cross-sectional area adjacent the inlet box, and thus narrows linearly in the direction of movement of the web, a well adapted pressure difference, also called delta P, is not obtained over the wire in the area at the dewatering space inlet end, which results in a defective build-up of the fiber web. Small fibers accumulate closest to the wire surface, and create a layer which is difficult for the filtrate to penetrate. The problem is to create a geometry for the dewatering table that provides an optimal pressure difference for the most effective dewatering and formation of the fiber web. The dewatering first begins some distance inside the dewatering space in the position where a first dewatering element is located. The inlet box comprises extended sealing blades whose free ends are arranged against the insides of the wires in the dewatering space. The ends of the sealing blades terminate at a position usually situated about 100 mm in advance of the location of the first dewatering element, which results in a relatively extensive backward flowing leakage of thick, fiber-rich flow occurs on both sides of the wires, where the ends of the sealing blades terminate.
One of the objects of the present invention is to provide an easier and improved adjustment of the pressure levels, and thereby more favorable dewatering, over the whole length of the dewatering table in a twin-wire press without changing the geometry of the dewatering table. Another object is to minimize leakage of fibers, particularly at the inlet end of the dewatering space.